For decades, users of internal combustion engines have struggled with engines that do not perform at levels at which the engines should be capable of performing, according to testing on models of those engines. For example, a diesel engine operating in normal working conditions may be less fuel efficient than what the engine is rated by a manufacturer or approved by regulatory bodies. Some experienced mechanics have succeeded in improving efficiency of some such engines, often through performing different maintenance actions in trial-and-error techniques that may be informed by the mechanic's experience and by the mechanic inspecting and testing the engine and its support systems.